Many of the chemical names referred today to as “common” or “trivial” — as opposed to “systematic” — at the time were very much systematic. Many of them, in fact, remain systematic because there is a system behind them.
Observe the structure (a):
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Its molecular formula, H2SO4, is probably the second most-known formula in the world after H2O. We can rewrite it as [SO2(OH)2]. There’s nothing easier than to create a completely systematic additive name for (a): dihydroxidodioxidosulfur. However, almost nobody uses this name because there is much more famous one: sulfuric acid. The Red Book lists it as an “acceptable common name” [1, Table IR-8.1].
From linguistic point of view, ‘sulfuric acid’ is a noun phrase consisting of the noun ‘acid’ and the adjective ‘sulfuric’. We came across noun phrases before: remember radicofunctional names such as ‘ethyl alcohol’? Here, too, we have the “specific” part (sulfuric) followed by the “generic” part (acid).
Semantically, acid in question is Brønsted acid, that is, hydron (H+) donor. Hence, a chemical name containing ‘acid’ implies there is at least one loosely bound hydrogen atom in the structure. ‘Sulfuric’ is more difficult to interpret. Sure, the root ‘sulfur’ gives us a clue: there is at least one sulfur atom. But what else? The -ic part is a common suffix that we see in adjectives derived from nouns. Therefore, ‘sulfuric acid’ means an acid somehow derived from sulfur.
So is the compound (b):
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Again, it could easily be named additively dihydroxidooxidosulfur. Its “acceptable common name” is sulfurous acid. The -ous bit is also a generic suffix used to form adjectives from nouns. In English, we say fantastic but fabulous, iconic but famous, ironic but humorous — there’s no apparent pattern as to why. However, there is one in the names of inorganic acids. We owe this system to a group of 18th-century Frenchmen.
‘Sulfuric acid’ and ‘sulfurous acid’ are the English calques of French acide sulfurique and acide sulfureux, respectively. These names appear in Méthode de nomenclature chimique (1787) in the following passage [2, p. 40]:
L’acide sulfurique exprimera le soufre saturé d’oxigène autant qu’il peut l’être; c’est-à-dire ce qu’on appelloit acide vitriolique. |
So, sulfuric acid is the one where sulfur is completely “saturated” with oxygen while sulfurous acid is the one where sulfur is combined, er, with a lesser quantity of oxygen. More generally, -ic became associated with higher oxidation state of the element in question, for instance, sulfur(VI) in (a), and -ous with lower oxidation state, e.g., sulfur(IV) in (b). But that’s it. These names don’t tell us what these oxidation states are. It’s impled that we already know them and can make an intelligent guess.
Other languages also adopted the French invention. In Romance languages, the adjective usually follows the noun, so, for example, Spanish ácido sulfúrico, Italian acido solforico and Neo-Latin acidum sulphuricum are all very similar: the adjective is formed by the suffix -ic- followed by the ending (inflectional suffix) that agrees with the head noun. In Russian, the noun typically follows the adjective, so we have серная кислота. Here, the Russian noun for sulfur, сера, was transformed into the adjective серная by the suffix -н- and the ending -ая. In German, as you might have expected, the two parts are glued together: Schwefelsäure is a single word, literally “sulfuracid”; the German word for sulfur, Schwefel, gets no suffix of any kind. Spanish also has its own word for sulfur, azufre, but it didn’t find its way in the name of the acid.
Similarly, acide sulfureux become ácido sulfuroso, acido solforoso and acidum sulphurosum in Spanish, Italian and Neo-Latin, respectively. In addtion to -н-, Russian сернистая кислота has got an extra suffix, -ист- (equivalent of English -y, “having the quality of”). Somewhat surprisingly, the German version now consists of two words, schweflige säure. The adjective is formed by the suffix -ig- (-y again) and ending -e*.
Lavoisier & Co. are also responsible for giving us the terms ‘sulfate’ and ‘sulfite’ (spelt in English exactly as in French) [2, p. 40]:
Sulfate sera le nom générique de tous les sels formés de l’acide sulfurique. |
Originally introduced as generic terms for salts of sulfuric and sulfurous acids, respectively, in modern usage they also became the names of the corresponding anions (c) and (d):
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Additive names of heteropolyatomic anions end with ‘ate’ [1, IR-5.3.3.4], so (c) will be named tetraoxidosulfate(2−) and (d) trioxidosulfate(2−). ‘Sulfate’ and ‘sulfite’ are much shorter and, when found on their own, pretty unambiguous. Still, in German and Russian their equivalents are often used in conjunction with the word ‘ion’ or ‘anion’, as in Sulfit-Ion or сульфат-анион, just to be on the safe side.
The ‘ic/ous/ate/ite’ approach was expanded to other oxoacids which were unknown in the times of Méthode. Consider (e) and (f):
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By analogy with sulfur oxoacids, the one with more oxygen atoms, or the higher oxidation state of iodine — that is, iodine(V) — is called ‘iodic acid’ (e), and the one with fewer oxygen atoms, or the lower oxidation state of iodine, viz. iodine(III), is called ‘iodous acid’ (f). The anions [IO3]− and [IO2]− are known as ‘iodate’ and ‘iodite’, respectively.
So far so good. But wait: there are other iodine oxoacids.
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We can think of the structure (g) as iodic acid (e) with one more oxygen atom attached to the central iodine atom. This compound is known as ‘periodic acid’. This name has nothing to do with periodicity: it is formed from ‘iodic acid’ with the Latin augmentative prefix per-, “very” (cf. alternative German name Überjodsäure). So if ‘iodic acid’ is the acid where iodine is “saturated” with oxygen, ‘periodic acid’ is the one where iodine is “supersaturated”. By analogy with iodate, the name of the periodic acid anion [IO4]− is ‘periodate’.
Similarly, we can consider the structure (h) a derivative of iodous acid (f) which lacks one oxygen atom. Its common name is ‘hypoiodous acid’, which is ‘iodous acid’ with the Greek-origin prefix hypo-, “below” or “under” (cf. the obsolete German name unteriodige Säure). As you may have guessed, the name of the hypoiodous acid anion [IO]− is ‘hypoiodite’.
Thus, in order of decreasing oxidation number for iodine, we have the sequence periodic acid > iodic acid > iodous acid > hypoiodous acid (periodate > iodate > iodite > iodite). The same naming pattern is used for chlorine and bromine oxoacids:
| Formal oxidation state | VII | V | III | I |
|---|---|---|---|---|
| Structural formula | [XO3(OH)] | [XO2(OH)] | [XO(OH)] | [X(OH)] |
| X = Cl | perchloric acid | chloric acid | chlorous acid | hypochlorous acid |
| X = Br | perbromic acid | bromic acid | bromous acid | hypobromous acid |
| X = I | periodic acid | iodic acid | iodous acid | hypoiodous acid |
Quite nice and systematic, isn’t it?
There is a peculiarity in Russian names for halogen oxoacids. Knowing that sulfuric acid = серная кислота, you’d expect iodic acid = иодная кислота. But no. The name иодная кислота is reserved for periodic acid. The next acid in the sequence, iodic acid, is иодноватая кислота, which contains the suffix -оват-, “-ish”, in addition to -н-. Iodous acid is иодистая кислота. This is the same -ист- as we’ve seen in сернистая кислота (sulfurous acid), but this time there is no -н- preceding it. Finally, иодноватистая кислота has got three suffixes, -н-, -оват- and -ист-, in a row. The sequence иодная > иодноватая > иодистая > иодноватистая is a music for the ear: one can almost taste the gradual dilution of “iodiness” caused by a play of suffixes. The same could be said of хлорная > хлорноватая > хлористая > хлорноватистая and бромная > бромноватая > бромистая > бромноватистая sequences. Somewhat confusingly, for the Russian-language chemistry students that is, the Russian names of the corresponding anions follow the “international” (i.e. Eurocentric) pattern, so the anion of иодная кислота will be периодат, etc.:
| Formal oxidation state | VII | V | III | I |
|---|---|---|---|---|
| Structural formula | [XO3(OH)] | [XO2(OH)] | [XO(OH)] | [X(OH)] |
| English | periodic acid | iodic acid | iodous acid | hypoiodous acid |
| Russian | иодная кислота | иодноватая кислота | иодистая кислота | иодноватистая кислота |
| Structural formula | [IO4]− | [IO3]− | [IO2]− | [IO]− |
| English | periodate | iodate | iodite | hypoiodite |
| Russian | периодат | иодат | иодит | гипоиодит |
In general, when it comes to anions, the picture is getting so regular it’s boring. Unique German and Russian roots found in oxoacid names vanish upon, well, ionisation: Salpetersäure → Nitrat, schweflige Säure → Sulfit, кремниевая кислота → силикат, мышьяковая кислота → арсенат and so on†.
But let’s come back to iodine oxoacids. I’ve chosen iodine over chlorine and bromine for a reason. Here it is (i):
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That’s right, yet another iodine oxoacid! Its additive name is pentahydroxidooxidoiodine, and acceptable common name is ‘orthoperiodic acid’. The prefix ortho-, from Greek ὀρθός, means “right”, “straight”, “true”. So ‘orthoperiodic acid’ means “true periodic acid” — as opposed to simply ‘periodic acid’ (g) which is, what, “false periodic acid”? Both (g) and (i) contain iodine in its highest oxidation state (VII) but differ in coordination number:
| Formal oxidation state | VII | VII | V | III | I |
|---|---|---|---|---|---|
| Coordination number | 6 | 4 | 3 | 2 | 1 |
| Structural formula | [IO(OH)5] | [IO3(OH)] | [IO2(OH)] | [IO(OH)] | [I(OH)] |
| Name | orthoperiodic acid | periodic acid | iodic acid | iodous acid | hypoiodous acid |
The only other acid where The Red Book still accepts the use of ortho- is orthotelluric acid (j) [1, Table IR-8.1]‡. “Simply” telluric acid (k) is the tellurium analogue of sulfuric acid (a). The formal oxidation state of both telluric and orthotelluric acids is VI.
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To sum up:
- The popular and time-proven method of naming mononuclear inorganic oxoacids and their anions is based on a system proposed in Méthode de nomenclature chimique [2] more than 200 years ago. IUPAC lists many of them as “acceptable common names”.
- The name of an oxoacid is a noun phrase consisting of the adjective followed by the noun ‘acid’.
- The suffixes -ic or -ous and prefixes per- or hypo- are used to construct the adjectives from (the roots of) the element names.
- This name does not tell us how many oxygen and hydrogen atoms are there.
- In the names of anions, the suffixes -at- and -it-, followed by the ending -e, replace the suffixes -ic and -ous, respectively.
- The prefix ortho- is introduced when there is a need to distinguish between oxoacids with the same formal oxidation state of the central atom. In this case, ortho- is assigned to the acid with the higher coordination number.
| * | Note the elision of the second ‘e’ of Schwefel in schweflige, although the alternative form, schwefelige, also exists. |
| † | We’ve seen the reverse situation, that is, irregular anion names, for several metals, e.g. gold → aurate, iron → ferrate, tin → stannate, etc. |
| ‡ | The Red Book says [1, p. 132]:
The prefix ‘ortho’ has not been used consistently in the past ... Here, it has been removed in the cases of boric acid, silicic acid and phosphoric acid where there is no ambiguity in the names without ‘ortho’. The only cases where ‘ortho’ distinguishes between two different compounds are the telluric and periodic acids (and corresponding anions).Why don’t we abandon those inconsistent prefixes altogether and introduce something completely different (in the context of chemical nomenclature), like arch- or ultra-? |
References
- Connelly, N.G., Hartshorn R.M., Damhus, T. and Hutton, A.T. Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005. Royal Society of Chemistry, Cambridge, 2005, p. 127—132, Table IR-8.1.
- Guyton de Morveau, L.-B., Lavoisier, A., Berthollet, C.-L., de Fourcroy, A.-F., Hassenfratz, J.H. and Adet, P.A. Méthode de nomenclature chimique. Chardon, Paris, 1787.
See also
- Acceptable common names of mononuclear oxoacids in English, German, Spanish and Russian.
- Acceptable common names of mononuclear oxoacid anions in English, German, Spanish and Russian.
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